Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02781533 2012-05-10
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A SHOCK DAMPING AND THERMAL ISOLATION SYSTEM FOR
A SURFACE MOUNTED VIBRATION SENSITIVE DEVICE
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims the benefit of U.S. Provisional Patent
s Application Serial No. 61/260,59 1, which was filed on November 12, 2009, by
Karoly Becze et al. for a A SHOCK DAMPING SYSTEM FOR A SURFACE
MOUNTED VIBRATION SENSITIVE DEVICE and is hereby incorporated by
reference.
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates generally to shock and/or vibration-damping apparatus
for providing a shock damping system to a surface mounted vibration sensitive
device, such as, a surface mounted temperature controlled crystal oscillator
to be
disposed in rugged/high shock/vibration environments.
Background Inforination
A surface mounted temperature controlled crystal oscillator (SMT-TCXO) is
an electronic circuit that uses the mechanical resonance of a vibrating
crystal of
piezoelectric material to create an electrical signal with a very precise
frequencies
over a wide ranges of ambient temperatures. This frequency source is commonly
used to provide a stable clock signal for digital integrated circuits, and to
stabilize
frequencies for radio transmitters and receivers and the like. One of the most
common types of piezoelectric resonator is a quartz crystal. Because they can
be used
over wide temperature ranges, SMT-TCXOs are ideal for applications that have
demanding timing specifications, such as GNSS applications for use in
surveying and
so forth.
These applications may require the use of SMT-TCXO based GNSS receivers
in environments in which there is a risk of high impact to the SMT-TCXO. For
example, during normal usage of a survey pole apparatus, a GNSS receiver may
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receive shock while placing a GNSS equipped survey pole at an intended survey
point. The crystals are mechanical resonators. Hence, they may exhibit a
sudden
phase shift adversely affecting performance of the GNSS receiver or other host
circuits. In addition to the extreme sensitivity of the crystals, the SMT-TCXO
itself is
s very small, on the order of 10mm. Thus, finding a system which provides
sufficient
damping properties while at the same time providing a continuous electrical
contact
has proven to be difficult at best.
In previous designs, vibration/shock isolators such as rubber washers or
spacers have been disposed between a housing and a printed circuit board (PCB)
which the SMT-TCXO is directly mounted via a soldering processes, in order to
absorb some of the vibration/shock related to the use of the device. However,
in this
system, the whole PCB is isolated which lends itself capable of minor relative
motion
with respect to the housing during vibration/shock events. Thus is, not ideal
because
the PCB and the SMT-TCXO are in direct contact with each other. It is also
difficult
to control heat transfer from the PCB to the SMT-TCXO when they are in direct
contact with each other. As known to those skilled in the art, heat can
adversely
affect the performance of the crystal, in ways that cannot be readily
determined, and
thus, compensated for, based on changes in ambient temperature. This phenomena
increases as the SMT-TCXO mounted PCB generates excessive heat.
The GNSS receiver tracking loops are extremely sensitive to oscillator phase
changes. Therefore, there remains a need for a shock damping system which
would
both absorb any shock that may be transferred to the device and at the same
time
protect the SMT-TCXO from heat transfer from the PCB.
SUMMARY OF THE INVENTION
The current invention overcomes the disadvantages of the prior art by
providing a shock damping and thermal isolation system for a surface mounted
temperature controlled crystal oscillator (SMT-TCXO) disposed in a rugged,
high
shock/vibration environment while providing stable electrical connection.
Specifically, an elastomeric material having a relative z-axis conductive
fiber material
incorporated within, may be disposed between a printed circuit board (PCB) or
other
electrically interconnecting mounting substrate and a SMT-TCXO. The
elastomeric
material is sized and positioned across the entire bottom surface of the SMT-
TCXO to
completely protect the SMT-TCXO from direct contact with the PCB. Once the
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elastomeric material is disposed between the PCB and the SMT-TCXO, the
components are compressed together with a component restraining system. The
component restraining system facilitates a secure electrical connection
between one or
more interconnect pads attached to the SMT-TCXO device and one side of the
s elastomeric material, and one or more interconnect pads attached to the PCB
and an
opposite side of the elastomeric material, while at the same time providing
protection
to the SMT-TCXO from vibrations that would otherwise be transferred to the SMT-
TCXO. The elastomeric material then allows signals and current to flow between
the
SMT-TCXO and the PCB via the z-axis conductive fibers, eliminating the need
for
attached wires or leads, such that mounting the SMT-TCXO is less complex.
The component restraining system secures the elastomeric material and SMT-
TCXO assembly to the PCB by applying a required amount of pressure to a top
side
of the SMT-TCXO opposite the PCB from the top side of a properly sized inner
cavity of the component restraining. The pressure provided from the top side
of the
inner cavity opposite the SMT-TCXO ensures a continuous electrical contact
between
the one or more interconnect pads of the SMT-TCXO and the one or more
interconnect pads of the PCB through the elastomeric material. Alternative
embodiments of the present invention may also dispose an additional or second
elastomeric material between the component restraining system and the SMT-TCXO
to provide additional damping for the SMT-TCXO and prevent damage and
vibration/shock transfer to the SMT-TCXO from the restraining system. The
additional elastomeric material, however, does not necessarily provide
electrical
connections to the SMT-TCXO, and thus, this material may, but need not include
conductive fibers.
By rigidly mounting (i.e. providing no relative motion) the PCB 120 and
isolating the SMT-TCXO 110, an optimal heat transfer path can be achieved as
the
main method of dissipating the generated heat via conduction through the PCB
120and directly into the housing 105.
The same shock damping system may be used with other surface mounted
devices for the same reasons and/or to provide damping of high frequency
vibrations
that would otherwise adversely affect the performance of the device.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention description below refers to the accompanying drawings, of
which:
Fig. 1 is an illustrative embodiment of the shock damping system for an SMT-
s TCXO device;
Fig. 2 is an illustrative embodiment of an exemplary elastomeric material
which may be advantageously used with the present invention;
Fig. 3 is an illustrative embodiment of an exemplary elastomeric material
which may be advantageously used with the present invention;
Fig. 4 is an illustrative embodiment of an alternative housing for the shock
damping system for the SMT-TCXO device;
DETAILED DESCRIPTION OF AN ILLUSTRATIVE
EMBODIMENT
Fig. 1 is an illustrative embodiment of the shock damping system for vibration
sensitive surface mounted devices, for example, a surface mounted temperature
controlled crystal oscillator (SMT-TCXO). Specifically, an elastomeric
material 115
having relative z-axis conductive regions, here, conductive fibers, separated
by non-
conductive regions,may be disposed between a printed circuit board (PCB) 120
or
other electrically interconnecting mounting substrate and an SMT-TCXO device
110.
The elastomeric material 115 is sized and positioned across the entire bottom
surface
of the SMT-TCXO 110 to completely protect the SMT-TCXO 110 from physical
contact with the PCB 120 and otherwise damp larger vibrations of the SMT-TCXO
that may be caused by, for example, impact. Once disposed between PCB 120 and
SMT-TCXO 110, all components are compressed together with a component
restraining system 105, to maintain alignment between component connect points
110 and PCB connect points 125 and provide a secure electrical connection
between
one or more interconnect pads 130 conductively attached to SMT-TCXO 110 and
one
or more interconnect/landing pads 125 conductively attached to PCB 120. The
elastomeric material 115 allows signals and current to flow between SMT-TCXO
110
and PCB 120 without the use of any attached wires or leads while at the same
time
providing a system for damping any shocks or vibrations that may be
transferred
through the PCB to SMT-TCXO 110. In addition, component mounting is simplified
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because the elastomeric material is only z-axis conductive, and thus,
component pad
alignment constraints are eased.
As mentioned above, a component restraining system 105 is employed to
apply a specific amount of pressure to the top side 145 of SMT-TCXO 110 that
is
s opposite the PCB 120, such that when affixed to the PCB 120 a required
compression
of the elastomeric material 115 is acheived to provide continuous electrical
contact
through the elastomeric material. This required compression is vendor specific
and
depends on the type of elastomeric material which is being used. In order to
provide
the required compression to the elastomeric material between the SMT-TCXO 110
and the PCB 120, an inner cavity (not shown) of the component restraining
system
105 is sized essentially to surround the SMT-TCXO 110 and the elastomeric
material
when the elastomeric material is in the desired compressed state. The
component
restraining system is affixed to the PCB 120 using screws 152, and the
pressure
exerted from the top side of the inner cavity opposite the SMT-TCXO 110 as the
bolts
1s are tightened essentially clamps the elastomeric material 115 between the
SMT-
TCXO 110 and the PCB 120 thereby provides the compression required to the
elastomeric material 115 to ensure a continuous electrical contact through the
elastomeric material between the one or more interconnect pads on the SMT-TCXO
and the one or more interconnect pads on the PCB.
Specifically, in the present invention, the proper size of the inner cavity is
directly related to the size of the SMT-TCXO, and in particular the thickness
of the
SMT-TCXO. The cavity is sized such that when the component restraining system
is
in place and affixed to the PCB 120, the top side of the inner cavity (not
shown),
which is opposite the SMT-TCXO, holds the SMT-TCXO to clamp the elastometic
material to the desired compressed state. In one embodiment of the present
invention,
a thickness or height of the inner cavity is illustratively determined by
applying the
formula:
Hcavity = H",cxo + Hcomp
In the illustrative formula, H,-cxo is the thickness of the SMT-TCXO 110, and
Hcomp is the thickness of the elastomeric material in the desired compressed
state. The
height of the elastomeric material at a desired compression is equal to Hõom(1-
D),
where Hõom is the nominal height of the elastomeric material 115 and D is the
percent
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deflection of elastomeric material, e.g., usually around 10-25% depending on
the type
of elastomeric material being used. Provided the inner cavity height is sized
according to the above calculation, the compression restraining system, when
affixed
to the PCB 120, will provide alignment and the required amount of compression
for a
s continuous electrical contact between the one or more interconnect pads on
the SMT-
TCXO and the one or more interconnect pads on the PCB
In alternative embodiments of the present invention, additional elastomeric
material 109 may also be disposed between component restraining system 105 and
SMT-TCXO 110 and/or on each of the sides of the SMT-TCXO along the x-axis and
to the y-axis. The optionally added elastomeric material does not necessarily
provide
any conductive purpose to the overall device, and may, but need not, contain
conductive fibers. The inclusion of the optional material provides additional
damping
and alignment to SMT-TCXO 110, thereby allowing even further protection to the
device against, for example, vibrations transferred through the component
restraining
15 system 105. In this embodiment of the present invention, the height of the
inner
cavity is illustratively determined by applying the formula:
Hcavity = H"icxo + Hnom cone(1-D1) + Hnom layer(1-D2)
20 In the above illustrative formula, HõQm co,,,, is the nominal height of the
conductive
elastomeric layer 115 and Hõom layer is the nominal height of the non-
conductive layer
109. Furthermore, D1 is the percent deflection of the conductive elastomeric
material
115, while D2 is the percent deflection of the additional material 109.
Although
additional material 109 may be the same elastomeric material as 115, it may
also be
25 any other material that has the same absorption properties. This material,
however,
need not have the same hardness, thickness or conductive properties as the
conductive
elastomeric material 115.
Illustratively, two or more screws are threaded through two or more apertures
155 in housing 105 and into corresponding apertures 155 in PCB 120, to secure
the
30 housing in place while at the same time compressing the elastomeric
material. In
alternative embodiments, however, adhesive or any other securing apparatus,
such as
direct solder, may be used to attach component restraining system 105 to PCB
120.Furthermore, although the above invention describes elastomeric material
105 as
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being a z-axis fiber conductive material, other variations of elastomeric
materials may
also be used. Figs. 2-3 illustrate exemplary alternative embodiment of the
elastomeric
material 105 which may be advantageously used with the present invention.
Specifically, Figs. 2 and 3 alternatively illustrate utilizing an exemplary
combination
s elastomeric material that has selective portions 206, 306 of the material
that are
conductive and other portions 208, 308 of the material that are non-
conductive.
Elastomeric material 205 illustrates a type of material that may be
advantageously used in place of elastomeric material 105 in the system of Fig.
1.
Material 205 is manufactured with alternating conductive and non-conductive
stripes
known by those skilled in the art as "Zebra Stripes." While assembling the
system,
the material 205 is positioned so that one or more of the stripes 206 that are
conductive are aligned with interconnect pads 130 and 125. As discussed above,
the
alignment of the conductive stripes and pads need not be precise, since the
conductive
stripes are disposed between non-conducting stripes 208.
Alternatively, combination material 305 of Fig 3 is selectively molded to
match the arrangement of the interconnecting pads 125 and 130, such that
conductive
material 306 extends between the pads and non-conductive material 308
surrounds the
conductive material.
Although the above described illustrative embodiment utilizes attached
mounting bracket and screws to secure the component restraining system 105 to
the
PCB 120, alternative embodiments, may utilize, adhesive or any other securing
apparatus, such as direct solder, to attach component restraining system 105
to PCB
120. For example, Fig. 4 illustrates an alternative embodiment of the present
invention where the screws 152 and the apertures 155 have been replaced with
twist
tabs 452. Shock damping system 400 utilizes the twist tabs 452 which slide
into slots
451 in the PCB 420 and are then tightly twisted 90 degrees over the edges of
the slots
451 on a side opposite 422 of the SMT TXCO 410 thereby securing the component
restraining system 405 in place while at the same time compressing the
elastomeric
material 409 and/or 410 against interconnecting pads 425 and 430.
Beneficially, this
embodiment allows users to quickly remove the component restraining system
405,
e.g., to replace various components of the above described shock damping
system.
Advantageously the current invention absorbs high shocks and vibrations
transferred to a SMT-TCXO device while at the same time alleviating concerns
with
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alignment and disruption of signals and electrical current from the SMT-TCXO
to an
interconnecting substrate or PCB in the event the SMT-TCXO device is subjected
to
excessive shock and vibration. For example, the above described component
restraining system aligns and constrains the elastomeric material so that the
material
s does not move between the SMT-TCXO 110 and the PCB 120 conductive pads 130
and 125 even when the device is impacted by an external force. Therefore, the
device
provides the user with a rugged impact resistant device that would be ideal
in, for
example, surveying environments. Furthermore, by rigidly mounting (i.e.
providing
no relative motion) the PCB 120 and isolating the SMT-TCXO 110, an optimal
heat
transfer path can be achieved as the main method of dissipating the generated
heat via
conduction through the PCB 120and directly into the housing 105.
Illustrative embodiment of the present invention may be implemented in a
GNSS receiver mounted on a survey poll that is subjected to shock forces in a
single
direction. The shock forces originate when the surveyor, carrying the poll,
jams the
tip of the survey poll into the ground, perhaps hitting a rock or other hard
surface.
The shock propagates up the length of the poll to the location of the GNSS
electronics
containing the vibration sensitive components, e.g., the SMT TXCO 110.
Illustratively, the PCB 120, elastomeric material 115, the SMT TCXO 110 and
component restraining system 105 are placed such that the direction of the
shock is
through the z axis, and therefore through the elastomeric material 115 to the
SMT
TCXO 110. In other words, the PCB is mounted horizontally with respect to the
vertical axis of the pole. In this configuration, the elastomeric material 105
need only
be placed under the SMT TCXO 110 because shock is not expected from any other
direction.
In other high vibration environments, such as for example a helicopter, high
vibration and shock waves can be expected from all directions. In these
environments, the vibration sensitive elements must be protected by
compressive
shock absorbing material from all sides. Accordingly, the elastomeric material
may
be placed between the vibration sensitive components, e.g., the SMT TCXO, and
the
incoming direction of the expected shock waves.
Furthermore, although the present invention has been described as being
implemented in a SMT-TCXO, the present invention may also advantageously be
applied to any vibration sensitive devices that may be adversely impacted by
vibrations, for example, inertial sensors, gyros, accelerometers, and so
forth.
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The foregoing description has been directed to specific embodiments of this
invention. It will be apparent, however, that other variations and
modifications may
be made to the described embodiments, with the attainment of some or all of
their
advantages. For instance, it is expressly contemplated that the assemblies,
systems,
and materials described herein may be implemented in various forms.
Furthermore,
in alternate embodiments, the optional second elastomeric layer 109 may
provide an
electrically conductive purpose. Therefore, Fig. 1 is provided as exemplary
only and
should not be construed to limit the claimed invention in any way.
Accordingly, this
description is to be taken only by way of example and not to otherwise limit
the scope
of the invention.
What is claimed is:
